Electric steel is a ferro-silicon soft magnetic alloy with extremely low carbon content, which is mainly used in manufacture of electric motors, generators, transformer iron cores and so on. Iron loss of the electric steel is a leading indicator to measure properties of the electric steel product. Iron loss mainly includes three portions, i.e., hysteresis loss, eddy current loss and abnormal loss. In subsequent processes, the non-oriented silicon steel is usually punched into core-shaped blanks, which, after being stacked up, are secured together by welding or riveting.
In general, surfaces of the non-oriented silicon steel sheet need to have an insulation coating so as to provide a high-level interlaminate resistance. This resistance can minimize eddy current loss of a stator, after the silicon steel sheets are punched and stacked to form the stator material. The insulation coating is usually called non-oriented coating. The non-oriented coating shall have a good adhesiveness, in order to ensure the insulation coating will not be peeled off from edges of the silicon steel sheets while they are punched into blanks. When being punched, slotted and/or cut, the insulation coating shall not pulverize excessively, so as to prevent some pulverized materials from accumulating on feed rollers or other devices to wear out moulds for punching or cutting the blanks. Meanwhile, the insulation coating shall allow welding the iron core at a reasonable rate.
The insulation coating applied on the surfaces of the non-oriented silicon steel can be divided into organic coating, inorganic coating and organic-inorganic composite coating, among which the organic-inorganic composite coating has both advantages of pure organic coating with good punchability and advantages of pure inorganic film with good heat resistance and weldability, and therefore can meet requirements of most users and thus is widely applied.
For example, U.S. Pat. No. 4,844,753 disclosed a solution for forming insulation coating on electric steel sheet, which consists of a film-forming component of inorganic chromate and a resin component. The resin component is a mixture of acrylic acid or acrylic acid-styrene copolymer emulsion and melamine resin of 0.2˜1 μm granularity. U.S. Pat. No. 4,618,377 disclosed an organic/inorganic coatings composition for forming insulation coating on surface of electric steel, which includes one type of organic resin, one type of resin particles that is improved with dispersant, and one type of solutions that contains at least one type of inorganic phosphate or chromate composite. Korean patent KR25106, 31208, 31219, U.S. Pat. No. 4,316,751, U.S. Pat. No. 4,498,936, and Japanese patent publication JP  50-15013 all disclosed a manufacture method of chromium-containing organic-inorganic insulation coating. However, the formulas in these patents all utilize oxides of chromium or chromates. The chromates, due to carcinogenicity, are restricted more and more severely for usage, as the requirements for environmental protection are increasingly strict.
In view of the above reasons, formula of chromium-free insulation coating for non-oriented silicon steel develops rapidly. Ordinary chromium-free semi-organic coating is introduced phosphates to take the place of bichromates. Phosphate coating has good insulativity and heat resistance, but trends to cause overwear to molds when punching stacked blanks, and thus reduce corrosion resistance and adhesiveness of the coating thanks to lack of chromates. For example, U.S. Pat. No. 2,743,203 disclosed that good insulation film can be obtained from a solution containing 7˜50% free phosphoric acid or from a phosphoric acid solution with dissolved magnesia therein. U.S. Pat. No. 4,496,399 refers to an inorganic/organic phosphate insulation coating. The inorganic part in its ingredients is aluminum and magnesium phosphate, colloid silicon dioxide and chromium acid anhydride, or a sort of aluminum silicate particles, while the organic part is emulsion of acrylic acid or ethylene acetate. Disadvantage of such coating lies in that the inorganic part contains free phosphoric acid, which will react with steel surfaces at high temperature, and magnesium phosphate has to be solidified at high temperature to prevent the blanks from adhering together, but resin trends to decompose, which renders the coating brown. Meanwhile, usage of chromium acid anhydride is not environment-friendly.
Japanese patent publication JP  2004-322079 disclosed a method for improving corrosion resistance of a coating by using compound phosphate with particularly proportioned aluminum phosphate, magnesium phosphate and calcium phosphate. Due to the usage of phosphate in such coating, excessive residual phosphoric acid therein is easy to cause adhesion of blanks. Japanese patent publication JP  11-131250 and Korean patent KR1999-26912 disclosed that siliane coupling agent is adopted to prevent interlaminate adhesion caused by residual phosphoric acid. Korean patent KR1999-26912 and Japanese patent publication JP  3370235 disclosed that silicon oxide sol, aluminum oxide sol, zirconia micro-particles, etc., are adopted to obtain non-oriented silicon steel insulation coating with high-corrosion-resistance and good-adhesiveness. These formulas with phosphates and inorganic sols as host agents still have the problem of interlaminate adhesion because of presence of phosphates, and the problem of low corrosion resistance because of presence of colloidal silicon oxide.
For chromium-free semi-organic coating using phosphate as host agent, for the reason of residual phosphoric acid, it is extremely prone to bring about interlaminate adhesion problem. European patent EP1208166B1 utilizes a insulation coating liquid with aluminum dihydrogen phosphate of 100 shares, acrylic acid-styrene emulsion of 28-98 shares, zinc nitrate of 6-18 parts by weight, silicon compound of 4-13 parts by weight, glycol of 18-35 parts by weight and non-ionic surfactant of 3-11 parts by weight, which presents water absorptivity since it has no stabilizer of residual phosphoric acid. Further, by using zinc nitrate, the presence of nitrate ions has remarkable adverse effects on corrosion resistance. In U.S. Pat. No. 5,955,201, inorganic silicate particles are employed as stabilizer for the residual phosphoric acid, so that the problem of interlaminate adhesion is effectively overcome. But, inorganic silicate particles can not dissolve into solution system, and thus trends to precipitate. They are hard to show effects in absence of fully mixing. The patent WO2008/016220A1 disclosed that by utilizing cobalt hydroxide and strontium hydroxide as stabilizer for residual phosphoric acid, the problem of interlaminate adhesion might be overcome as well, but the two are inorganic particles that are hard to be mixed well.
At present, the chromium-free coatings produced and used in China have a less portion of inorganic ingredients. Although such coatings have a good-looking appearance and good punchability, the high temperature resistance thereof is poor, so that, after a high temperature treatment, interlaminate current resistance and insulativity of the coatings will be lowered greatly, and the coatings are easy to carburize, resulting in degradation of product performance.
Non-oriented silicon steels are divided into so-called white sheets and black sheets, depending on whether the steels are stress-relief annealed. In application fields of non-oriented silicon steel, such as medium and small-sized electric machines and EI (electric instrument) sheets, it is required to have the coatings being stress-relief annealed, to reduce iron loss of the silicon steel sheets and to improve electrical properties. Users of these fields usually put forward such a requirement that the coating of the non-oriented silicon steels present a raven-but-brilliant appearance after the stress-relief annealing process. If the coatings are lusterless after being stress-relief annealed, they are considered to have been improperly annealed and have poor insulativity, which then will be prone to be rejected by downstream users. The above-mentioned chromium-free coatings are all lusterless after being stress-relief annealed, and therefore fail to meet user's requirements.
Therefore, it is especially required to develop a semi-organic environment-friendly insulation coating for cold-rolled non-oriented electric steel to provide the non-oriented silicon steel sheets with good surface resistivity, lowered eddy current loss and good moisture absorption resistance, together with good adhesiveness, so that the coating will not pulverize during being cut into strips and punched to protect moulds; and the coating will not generate excessive air holes during welding process and not react with various refrigerants and frozen engine oil used in compressor; and the coating will not generate harmful matters, such as sexavalent chromate, during production and operation.
EU Commission and European Parliament issued two standard directives on Feb. 13, 2003, i.e., the directive on wastes of electric and electronic equipment (WEEE) and the directive on restriction of harmful substances (ROHS) in electric and electronic equipment, which require that index of harmful substances in electric and electronic products sold in the EU market must conform to relevant prescriptions from Jul. 1, 2006 onwards. The restricted harmful substances include: cadmium, lead, mercury, sexavalent chromate, multi-bromine biphenyl, polymerized biphenyl bromate and ether, etc.